ok, first, this idea is not from me. this skill has been made by brother laz in the median mod. I am just explaining how to do it.
i will explain you how to make a stationary avatar that casts some frost novas.

to follow this tutorial, you have to know how to use the needed tools.I also recommend to read the file guides of skills.txt, missiles.txt and skilldesc.txt

first, we will have to extract an animation in d2data.mpq because we will use the summoner animation as avatar.
so open d2data.mpq, using winMPQ and extract the following files:data\global\monsters\SU\TR\SUTRLITSCHTH.dcc

now the extracted file in your missiles folder (data\global\missiles\)
rename it to statavatar.dcc

now we can start to make our avatar skill. first open missiles.txt and scroll down to frozenorb line.
copy frozenorb line and paste it at the bottom, make these changes: